1. Field of the Invention
This invention relates to an apparatus and method which reduces transient exponential noise in sinusoidal signals and has particular application in reducing such noise in the signals processed by protective relays in an electric power transmission system. More specifically, the invention is directed to an apparatus and method which approximates the noise as a linear function and extracts the slope and initial ordinate value of that approximate linear function from sets of digital samples of the composite signal one-half cycle apart.
2. Background Information
Step changes in electrical circuits having reactive components fed by sinusoidal currents create transient responses which make it difficult to initially determine the new steady state conditions. One application where this phenomenon has particular significance is in protective relays used to protect electric power generating equipment, transmission lines, and transformers from destruction during fault conditions. These relays monitor the power line voltages and currents and make decisions based upon the values and relationships of the inputs. The decision making process is complicated by the presence of a decaying exponential current waveform during the first few cycles of the fault current, just when the protective relays must perform their function.
The presence of such transients in many instances is accommodated for by the use of conservative margins in the decision making process. For instance, in distance measuring relays which calculate the apparent impedance to a fault from the relay by dividing the measured voltage by the current to determine if the fault is within the protection zone of the relay, the reach characteristic with which the apparent impedance is compared is adjusted to take the transient into account. This reduces the accuracy of the calculation, however.
U.S. Pat. No. 4,577,279 discloses a technique for reducing the exponential transient noise in the signals used by a fault locator. This technique recognizes that the transient can be resolved into the sinusoidal power component and a decaying exponential and that if the signal is averaged over a full cycle, the sinusoidal component is eliminated. Based upon the assumption that the average value of the exponential occurs near the midpoint of the cycle, a running average of digital samples for the last full cycle is maintained and is subtracted from the value of the sample at the midpoint to determine the compensated digitized value of the sinusoidal signal at that point. A new running average is calculated for each new sample. The difficulty with this technique is that a complete power cycle is required to obtain the first compensated value and two full cycles are required to obtain a full compensated cycle of the sinusoidal signal. This delay is acceptable in applications like fault location, but is unacceptable in making trip decisions.
In another technique for removing a transient from a sinusoidal signal, samples are taken asynchronously one-quarter of the power cycle apart with the first and fourth samples being added and the second and third being subtracted. This digital filter removes the dc and ramp components leaving only higher order terms of the exponential transient which are said to be insignificant. One difficulty with this technique is that it requires one full cycle before compensated values are available. It is also not compatible with the use of a Fourier algorithm for deriving the sinusoidal signal from digital samples which requires a higher sampling rate for suitable accuracy.
There remains a need, therefore, for an improved method and apparatus for reducing transient exponential noise in a sinusoidal signal. More particularly, there remains a need for such a method and apparatus which is rapid and provides compensated data at a rate which can be used with a notch filter employing a Fourier algorithm for deriving the sinusoidal signal from digital samples.